The 6 Plus is the first iPhone that sports a “Retina HD” display — the sharpest display Apple has ever made. It forces designers to provide an additional set of image resources to developers to match that sharpness.

We needed only one set of assets for the original iPhone up to iPhone 3GS. And when iPhone 4 came out with the Retina display, we also needed 2x assets — images twice as detailed. Now, with the display of the 6 Plus being even more detailed than that of the iPhone 4, we will also need to provide 3x assets. The numbers 1x, 2x and 3x are also called “scale factors.”